The synthesis of various multi-arm radial or star polymers have become of growing practical and theoretical interest to a variety of industries. Such star polymers are seen as useful as, inter alia, surfactants, lubricants, rheology modifiers, and viscosity modifiers. In fact, star polymers are now considered by many to be state-of-the-art viscosity modifiers and oil additives, although the potential of some of these star polymers for these applications is still being evaluated and tested.
It is well known that star polymers containing polyisobutylene arms of controlled length and hence, molecular weight, can be synthesized by living cationic polymerization. For example, Kennedy et al. U.S. Pat. No. 5,395,885 describes the synthesis of star polymers consisting of multiple polyisobutylene (PIB) arms and a polydivinylbenzene (PDVB) core. These star polymers suffer from the disadvantage that the core is "ill-defined". By the term "ill-defined" it is meant that the core of the star polymer, e.g., PDVB, is an uncontrolled, crosslinked, gel-like structure having unsaturation sites in the core. In comparison, "well-defined" cores are built of readily characterizable, soluble molecules which are precursors to the core. As a result, the structure of the resultant star polymers having well-defined cores can also be controlled.
In particular, star polymers having well-defined cores are believed to impart better resistance to mechanical/chemical degradation than star polymers having ill-defined cores. Examples of polyisobutylene star polymers prepared with such well-defined cores include those stars having polyisobutylene arms emanating from calixarene cores, cyclic condensation products of a p-substituted phenol and formaldehyde, and cyclosiloxane cores, both of which have been shown to provide superior properties as compared to polyisobutylenes emanating from PDVB cores. To date, patents containing polyisobutylene arms emanating from these well-defined cores include Kennedy et al. U.S. Pat. No. 5,663,245, using polysiloxane cores and Kennedy et al. U.S. Pat. No. 5,844,056, using calixarene cores.
Beyond the advantages described hereinabove, it is believed that resultant star polymers having polyisobutylene arms having well-defined cores, and particularly, cyclosiloxane cores as set forth in U.S. Pat. No. 5,663,245, will be more acid-base stable than other siloxane-containing compounds. For example, it is well known that Si--O groups are easily hydrolyzable in the presence of strong acids. That is, it is believed that the polyisobutylene arms of the subject polymer protect the Si--O groups from hydrolytic attack, thereby aiding in the acid-base stability of the polymer.
Cyclosiloxanes (D.sub.n.sup.H) are also seen as another potential solution to the existing problem of shear stability posed by star polymers having ill-defined cores. Silicone oils have long been demonstrated to have superior shear stability properties as compared to hydrocarbon oils (Fitzsimmons et al., in Trans. ASME 68,361 (1946)), leading Kennedy et al., in U.S. Pat. No. 5,663,245, to purpose the synthesis of first order stars having multiple arms of polyisobutylene (PIB) emanating from a cyclosiloxane (D.sub.n.sup.H) core.
By the term "first order" it is meant that the star polymers are essentially synthesized by linking, through hydrosilation, a number of olefin-terminated polyisobutylene prearms to the Si--H groups of a single cyclosiloxane molecule. In theory, the number of arms emanating from a first order star molecule will total the number of Si--H groups on the cyclosiloxane molecule. For example, in U.S. Pat. No. 5,663,245, Kennedy et al. describes a first order star with a hexamethylcyclosiloxane molecule (D.sub.6.sup.H) core having six Si--H groups, where a maximum of six polyisobutylene arms radiate from the molecule. These first order stars were formed by hydrosilation of, inter alia, .omega.-allyl-terminated polyisobutylene (PIB-CC.dbd.C) with methylcyclosiloxanes carrying 4 to 8 Si--H groups (D.sub.n.sup.H, where n=4 to 8).
The formation of higher order stars was also first described by Kennedy et al., in U.S. Pat. No. 5,856,392. In the course of investigating the multiple PIB-arm/cyclosiloxane core first order star polymers using D.sub.6.sup.H, it was. observed by Kennedy et al,. that in the presence of trace amounts of water in the hydrosilation charges, a small number of stars had a higher number of arms than the expected six as the result of random core-core coupling.
"Higher order" stars may be envisioned as clusters of first-order star polymers linked together by their siloxane cores. Theoretically, the core component of a "higher order" star polymer has many more Si--H groups available for linkage to olefin-terminated polyisobutylene prearms. This is true because the core component in higher order stars includes a plurality of individual cyclosiloxane molecules coupled together, and only one reactive site is needed to couple two cyclosiloxanes, leaving many times more Si--H groups available for linkage than are present on any one individual cyclosiloxane molecule.
The present invention continues and improves this art relating to star polymers having polyisobutylene arms radiating from a well-defined siloxane core. In particular, it will be appreciated that both U.S. Pat. No. 5,663,245 and U.S. Pat. No. 5,856,392 focus on the synthesis of the polyisobutylene arms to a siloxane core, whether the core be one siloxane molecule (i.e., U.S. Pat. No. 5,663,245) or several (i.e., U.S. Pat. No. 5,856,392) in the formation of "first order" and "higher order" stars, respectively. Neither patent suggests that the prearms of polyisobutylene could instead be polymerized diblock prearms of isobutylene and another monomer connected to the opposite end of the polyisobutylene chain from the core. More importantly, the addition of certain monomers, including styrene, provides an entirely new class of compounds, i.e., star-block copolymers. Even more notable is that these compounds may be thermoplastic elastomers (TPEs) rather than simply viscosity improvers like the prior star polymers. It is believed highly desirable to provide star-block TPEs which are believed to have superior mechanical, thermal and Theological properties as compared to linear triblock TPEs or similar hard/soft segment compositions, including improved tensile strength and processing properties. Such star-block copolymers could also be synthesized as "higher order" star-block polymers in a manner similar to that described in U.S. Pat. No. 5,856,392.